The present invention relates to a toner for use in an image forming machine such as an electrophotographic recording machine or an electrostatic recording apparatus, and, more particularly, to a toner which is excellent in chargeability, charge rising properties and environmental stability.
In general, a toner used in an electrophotographic copying machine or an electrostatic recording apparatus comprises a binder resin, a colorant, a charge controlling agent and other additives. For the purpose of imparting desired charging characteristics, temporal stability and environmental stability to a toner, various charge controlling agents have been proposed. Examples of such charge controlling agents include a 2:1 type metal complex dye of an azo dye, a metal complex compound of a salicylic acid derivative (disclosed in Japanese Examined Patent Publication No. S55-42752, Japanese Laid-Open Patent Publication No. S61-69073, Japanese Laid-Open Patent Publication No. S61-221756, Japanese Laid-Open Patent Publication No. H09-124659 and so on) a metal salt compound of an aromatic dicarboxylic acid (disclosed in Japanese Laid-Open Patent Publication No. S57-111541, Japanese Laid-Open Patent Publication No. H07-295298 and so on), a metal complex compound of an anthranilic acid derivative (disclosed in Japanese Laid-Open Patent Publication No. S62-94856 and so on), and an organic boron compound (disclosed in Japanese Examined Patent Publication No. H07-31421, Japanese Examined Patent Publication No. H07-104620 and so on).
Those charge controlling agents, however, are heavy metal compounds the use of which is expected to cause environmental concerns or have drawbacks such as insufficient charge imparting properties, insufficient environmental stability and insufficient charge rising properties, so that none of those have satisfactory properties as a charge controlling agent.
The present invention has been made in view of the prior arts and it is, therefore, an object of the present invention to provide a toner which contains a charge controlling agent free from a heavy metal that may cause environmental concerns, and which is excellent in chargeability, charge rising properties and environmental fluctuation resistance.
In accordance with one aspect of the present invention, there is provided a toner comprising at least a binder resin, a colorant, and a product of an iron compound containing an aromatic hydroxylcarboxylic acid as a ligand and represented by the following formula (I): 
wherein R1, R2, R3, and R4 are independently selected from the group consisting of a hydrogen atom, a straight-chain alkyl group and a branched unsaturated alkyl group, and R1 and R2, R2 and R3, or R3 and R4 may define an aromatic ring which is fused to the benzene ring of the formula (I) and which may have a substituent, m is an integer at least 3 and n is an integer at least 2,
said product having such a characteristic that a filtrate obtained by filtering a dispersion of 10 g of said product dispersed in 200 ml of purified water provides an electric conductivity of not greater than 600 xcexcS/cm.
In another aspect, the present invention provides a method of preparing a toner, comprising the steps of:
providing a product of an aromatic hydroxylcarboxylic acid ligand-containing iron compound represented by the following formula (I): 
wherein R1, R2, R3, and R4 are independently selected from the group consisting of a hydrogen atom, a straight-chain alkyl group and a branched unsaturated alkyl group, and R1 and R2, R2 and R3, or R3 and R4 may define an aromatic ring which is fused to the benzene ring of the formula (I) and which may have a substituent, m is an integer at least 3 and n is an integer at least 2,
purifying said product to obtain a purified iron compound of the formula (I) having such a characteristic that a filtrate obtained by filtering a dispersion of 10 g of said purified iron compound dispersed in 200 ml of purified water provides an electric conductivity of not greater than 600 xcexcS/cm,
kneading said purified iron compound with a binder resin and a colorant, and pulverizing said kneaded mixture into powder.